Lift-off processing is a well-known, ubiquitous microfabrication technique that typically uses one or more layers of photoresist to define a pattern for selectively placing metal on a sample surface. See, e.g., U.S. Pat. No. 4,814,258 to Tam, entitled “PMGI Bi-Layer Lift-Off Process.”
Aspects of conventional lift-off process are illustrated in FIGS. 1A-1C. In the first steps of such a conventional lift-off process, illustrated in FIG. 1A, one or more layers of photoresist are deposited on a substrate and are masked with a photomask, with UV light being used to expose a pattern that can be developed (removed) using standard wet-chemical developers. After the patterned features are developed, as shown in FIG. 1B, metal is deposited across the entire sample, using a technique such as evaporation or sputtering. The act of “lift-off” occurs next, when a solvent or photoresist stripper is used to remove the remaining undeveloped photoresist, which carries with it the overlying (unwanted) metal, and as shown in FIG. 1C, where photoresist had been developed away, metal remains adhered to the sample surface in the desired pattern.
While such conventional photolithography-based lift-off processing has been the work-horse of the silicon industry for decades, it has limitations that prohibit it from being used to lift-off materials other than metals. The primary disadvantage of photolithography based lift-off is that it relies upon the use of polymer-based photoresist, which has a relatively low glass transition (melting) temperature typically between 100-150° C. Since several dielectrics and semiconductors require deposition/growth temperatures greater than 300° C. to achieve high electrical quality, photoresist lift-off is not a viable method for patterning them.
Another common disadvantage of photolithography is that photoresist developers are typically strong bases. In addition to environmental concerns about disposal, developers have been shown to undesirably etch certain metals and dielectrics. For example, commonly-used tetra-methyl ammonium hydroxide (TMAH) based developers, such as CD-26, is known to etch both aluminum metal and amorphous Al2O3. See S. Sirviö, R. L. Puurunen, and H. Kattelus. “Electrical Properties of Capacitors with ALD-Grown Al2O3 and Al2O3—TiO2 Nanolaminate Thin Film Dielectric Layers.” Presented at AVS 8th International Conference on Atomic Layer Deposition. Bruges, Belgium. Jun. 29-Jul. 2, 2008. We have also experimentally observed amorphous Al2O3 etching in the sodium hydroxide-based developer 352.
Up until now, there has been only limited investigation into the use of non-photoresist materials for lift-off purposes. For example, Lehrer and Vincak used a bi-layer of SiO2/GeO2 patterned by photolithography and etched with a CF4 plasma etch. See U.S. Pat. No. 4,387,145 to Lehrer et al., entitled “Lift-off shadow mask.” Since the GeO2 etch rate is 5-10 times faster than the SiO2, this method creates a SiO2 overhang (referred to as a re-entrant profile) that serves to create a discontinuity in metal that is blanket deposited. After metal deposition, the inventors submerse the sample in water, which dissolves the GeO2 and lifts off the overlying layers. A limitation with this method is that the GeO2 and SiO2 are deposited in a conventional chemical vapor deposition (CVD) reactor at 400° C. with oxygen as a precursor. This could be deemed undesirable if a sample has an oxygen-reactive surface or if there are metal contacts present that would oxidize at 400° C.